Sunday, June 22, 2014

Why the Summer Solstice isn't the start of summer

This weekend was (in the northern hemisphere) the summer solstice – the "official start of summer", also known as the "start of astronomical summer", "the first day of summer", and "midsummer's day". Only the last of these makes any sense.

This idea seems to have grown out of a human desire to have major transitions well marked. It reminds me of something I once saw that divided the day into four parts: morning, afternoon, evening, and night. The transitions were dawn, noon, sunset, and midnight. That's right – this system had the 'night' starting at midnight.

There are at least two reasons why claiming the start of summer occurs at the summer solstice is wrong. The first is common sense, the second is science.

From a common sense perspective, we know when summer is – it's when the evenings are long and the days are hot. Similarly, winter is marked by cold days and early evenings. Yet 'astronomers' tell people shovelling snow off their driveways in mid-December that winter doesn't start for another week, tell sunbathers in early June that it isn't summer yet, and claim it is still summer as the equinoctial gales roll in. The message this sends out is that astronomers don't know much about the real world.

The meteorological summer covers June, July, and August and pretty much matches our perception of what is 'summer'. This corresponds to the Memorial Day to Labor Day definition of summer in the US, for instance. Similarly the first of March – the start of meteorological spring – is when the daffodils are supposed to bloom for St David's day. Meteorologists clearly know about the real world.

So what about science? Saying the summer solstice is the start of astronomical summer implies there is something scientific going on. Is there a good reason why the astronomical seasons lag the meteorological and cultural seasons?

Sadly, nothing could be further from the truth. The Sun is responsible for the seasons, but the Earth takes time to warm (or cool), so the meteorological seasons actually lag behind their astronomical driver. The opposite of what we tell people.

This lagging is the reason why midsummer's day does not fall in the middle of the meteorological summer, but precedes it by about three weeks. The astronomical seasons should (if we insist on four seasons) start on the cross-quarter days. Now widely neglected, but well known in the past, these fall midway between the solstices and the equinoxes and are marked by traditional festivals.

The exact dates of the astronomical cross-quarter days for 2014 (from archaeoastronomy.com) are 3rd February, 5th May, 7th August, and 7th November. These correspond (approximately) to the traditional cross-quarter days of Candlemass (a.k.a Groundhog Day), May Day, Lammas, and All Hallows in the Christian tradition; or Imbolc, Beltane, Lughnasadh, and Samhain in pre-Christian Celtic tradition.

These dates tie in to genuine science about how the Earth is heated by the Sun. That they fall a bit before the meteorological seasons is explained by the lag in the Earth's response to this heating – effect lags behind cause. When people say in late August that it is still summer, we can answer that the Sun has already moved into autumn, but the Earth's captive heat means we still experience summer weather. The disconnect from people's experience can now be explained with real physics.

So, I hope you had a good midsummer's day, but don't go calling it the start of summer. Astronomically and meteorologically, summer has already been going for quite a while.